North and South Korea exchanged accusations after navy ships from each side opened fire on the other, raising tensions along their contested maritime border.

North Korea on Friday angrily denied its navy fired on South Korean ships near their de facto sea border, and instead claimed it was the victim of preemptive shelling.

South Korea's Joint Chiefs of Staff said a North Korean ship fired at a patrol boat near the disputed border, known as the Northern Limit Line, on Thursday. The JCS said the boat was not hit and returned fire that also did not hit anything.

Pyongyang's Korean Central Broadcasting Station on Friday read out a statement from the Korean People's Army saying its ships and Chinese ships were the ones shot at.

The newsreader said warships of the South Korean “puppet navy” intruded deep into the waters of the North Korean side. She said the incident happened on the North’s side of the maritime demarcation line and was a preemptive firing under the pretext of intercepting peaceful Chinese fishing boats. She said South Korea is trying to convince the public that the KPA was responsible for the shelling.

South Korea's Ministry of Unification said it is rare for North Korea to make such a blatant denial of a firing incident.

Defense Ministry Spokesman Kim Min-seok said they received a letter Friday from North Korea formally denying any aggression.

Through the letter, he said, North Korea argued that yesterday's firing provocation against the ship near Yeonpyeong island was unrelated to them. Kim said this unreasonable argument is a shameless lie to avoid responsibility for the provocation and a laughing stock for the international community.

The KPA statement on North Korean radio said it was preparing to be fully combat-ready and would “mercilessly wipe out” what it called “puppet military hooligans keen on provocation.”

South Korean Defense spokesman Kim said they sent their own letter to North Korea gravely warning it against incitement.

He said South Korea has clearly expressed they will firmly respond if North Korea continues to provoke. The South Korean military has strengthened its surveillance and operation posture, he said, and maintains firm readiness against North Korea's various provocations.

The alleged exchange of fire raises tensions along the Northern Limit Line, which was established under United Nations Command but North Korea refuses to recognize.

“The South has adopted that and claims that is a maritime border. But, strictly speaking, according to international law it's not. In fact, there is no maritime boundary in the area so that's why there's this position from the North. They don't recognize and contest this line. So, it always has the potential for some flare-up in conflict,” said Daniel Pinkston, Deputy Northeast Asia Director with the International Crisis Group.

South Korean authorities evacuated residents of nearby Yeonpyeong island to bomb shelters. North Korea shelled the island in 2010 in response to military exercises, killing two soldiers and two civilians.

Earlier this week, the Joint Chiefs of Staff said South Korea's navy fired warning shots at three North Korean ships that crossed the maritime border.

North Korea claimed it was policing illegal Chinese fishing boats.

Pyongyang responded by threatening to strike at all South Korean ships that enter the sensitive waters.

The sinking of a South Korean navy vessel near the maritime divide in 2010 killed 46 sailors and was blamed on a North Korean torpedo. Pyongyang denies it was responsible.